Because of recent developments in power semiconductor devices such as power MOSFETS and IGBTs, the proliferation and usage of brushless D.C. motors has intensified in recent years. Their applications are centered around either variable/adjustable speed or servo positioning systems. The availability of high energy permanent magnets such as samarium cobalt or neodymium boron iron has also contributed to the current interest in brushless D.C. motors. Due to the high cost of these high energy magnets and mechanical difficulties of retaining them in mountings, however, there has also been a keen interest in the class of brushless D.C. motors that do not use permanent magnets or windings in connection with the rotating member. This class of brushless D.C. motors is commonly called switched reluctance motors or simply SR motors.
SR motors have been used extensively as stepping motors when driven by a series of clock pulses in an open loop manner such that they provide a commutation frequency and phase that is without regard to the angular position of the rotor. In these stepping motor systems, the motor has typically been referred to as a VR motor, where the "VR" is an acronym for the phrase "variable reluctance". Many of these so-called VR stepping motors are either three-phase or four-phase machines with laminated designs that include many teeth on each rotor and stator magnetic pole. These teeth are required to achieve small step angles. (e.g., U.S. Pat. No. 3,866,104 to Heine).
The subject of this invention has to do with a continuously rotating reluctance machine or SR motor that is not designed to be used as a stepping motor and controlled in an open-loop fashion. The SR motor in general is designed to convert electrical energy into a continuous mechanical rotation. This means that the SR motor is called upon to produce continuous torque at any desired, preset or controllable speed of rotation.
SR motors of the type described herein usually have stators wound with either three, four or five phases. Each phase is energized or connected to a DC power source and commutated or switched at the optimum position of the rotor, so as to produce the maximum output torque per phase. Torque variation or torque ripple can be minimized by careful commutation utilizing two phases energized at a time so the added torque of each adjacent phase approximately equals the peak torque for conventional SR motors although normally commutated with unipolar DC voltage with one phase on at a time. Although not a requirement, SR motors do not normally have multiple teeth per pole in their lamination designs but usually single poles for each coil. Two or three teeth per pole might be used occasionally but seldom more.
The SR machine is a very robust or rugged motor with a very simple rotor construction and an extremely compactly wound stator, which yields the lowest potential manufacturing cost of any motor known. They are well suited to heavy duty use in the most severe of environments. For example, they are capable of temperature extremes not possible with permanent magnet motors (between -100.degree. and +500.degree. C.)
SR motors have unique features that make it very attractive for certain applications where human safety is of utmost importance, such as in the aerospace and ground transportation industry. These features are the result of the SR motor not using permanent magnets. For example, in SR motors there is no need for bi-polar current to energize each phase in order to produce torque. The stator poles magnetically attract soft iron rotor poles rather than north or south magnetized permanent magnets, resulting in no need to control the polarity of the current used for each phase. Because polarity of the current is not important, the winding is connected in series with the switching transistors, thereby eliminating the possibility of shoot-through faults in the event of a switch failure as in the case with induction motors and permanent magnet brushless motors.
The other safety-related advantage of SR motors is also due to the lack of permanent magnets. The permanent magnets generate a back electromagnetic force (EMF) during rotation of the rotor. SR motors are without this type of back EMF. This fact eliminates the possibility of a braking torque on the motor shaft in the event of a short in the circuit due to the magnetomotive force (MMF) of the magnets. In the case of a short in one of the windings, the SR motor will continue to rotate, but it will produce reduced power. The percentage of reduced power is proportional to the number of open or shorted phases relative to the total number of phases.
One of the disadvantages of increasing the number of winding phases of any electric motor is the increase in switching or commutation frequency. When a phase is energized or de-energized the time rate of change of current (dI/dt) causes eddy current losses in the lamination iron of the stator and rotor causing heating. The faster the motor rotates the greater the frequency and iron core losses.
Another loss resulting from magnetic field flux reversals is known as hysteresis loss. This heating affect also increases with the number of phases and the rotational speed. A full magnetic flux reversal from some positive flux value to some negative flux causes a "full loop" energy loss. If the flux field only increased from zero to some maximum value then when commutated "off", the field decreases back to zero again then a "minor" hysteresis loop is produced. This phenomenon of hysteresis loss and corresponding heating affects takes place during the operation of most all electric motors. The so called iron losses of all motors produces heat which must be dispatched, either through the mechanical mounting structure by conduction or by means of air or liquid cooling. The area inside the hysteresis loop is the work done or energy loss. When a full magnetic flux reversal takes place due to bipolar current switching used in induction motors and brushless motors, the magnetic iron experiences heating due to a full hysteresis loop. The SR machine experiences the losses produced by the minor hysteresis loop because the flux starts at zero, increases to the peak value then decreases back to zero rather than to a minus peak value, but the resulting iron losses are much less than in conventional SR machines.
The SR motor illustrated in my U.S. Pat. No. 4,883,999 significantly reduces the energy losses experienced in the back iron and rotor of an SR motor. Although the torque performance is very good for such SR motors, they suffer from the same limitations of conventional SR motors in that there is no clear way to drive the motor with two phases on at the same time. Such a drive scheme is highly desirable for high torque application and/or applications that require continuous positive torque in various failure modes--e.g., aerospace industry, for safety reasons. In the past, phase overlap provided added torque and added safety. When two phases are applied to a conventional SR motor in synchronism, however, the motor fails to achieve greater torque and essentially becomes a very inefficient machine (i.e., twice the input power, but no significant increase in torque), which is due to magnetic saturation of the stator yoke and a stator and rotor pole distribution that is designed for single-phase energization and ineffective for two-phase energization.